Magnetization reversal mechanism of ramified and compact Co islands on Pt(111)

Abstract

We report on the magnetization reversal mechanism of Co islands on Pt(111) as a function of their size and shape. We measure the zero-field susceptibility χ (T ) and low-temperature magnetization curves M(H ) with in situ magneto-optical Kerr effect. Together with the island morphology deduced from scanning tunneling microscopy, this creates sufficient information to determine both the magnetization reversal mechanism and the distribution of anisotropy energies between perimeter and surface atoms. We find a transition from quasicoherent rotation to domain wall nucleation and propagation with a critical size of 350 atoms for ramified, and of 600 atoms for compact islands.

Spatially Resolved Magnetic Anisotropy of Cobalt Nanostructures on the Au(111) Surface

“…In surface-supported nanostructures, the magnetic anisotropy energy (MAE) can be modified in a controlled manner by varying the particle size, shape, and choosing appropriate substrate for inducing interface-driven effects. , The interfaces, in particular, combining a 3 d ferromagnet with its large spin moment and 4 d or 5 d metal with its large spin–orbit coupling (SOC) offer a fertile platform to explore the mechanism of enhancements of MAE. − Accounting for the precise structure-magnetic property correlation is essential to tune the MAE in such systems. Spatially averaging techniques, namely, magneto-optical Kerr effect (MOKE), and X-ray magnetic circular dichroism (XMCD), have been remarkably successful in revealing the magnetic properties such as magnetic moments and MAE in a monodisperse ensemble of surface-supported nanostructures. ,, However, for ensembles containing several species, unveiling the structure–magnetic property correlation, including the interface effects, demands a spatially resolved magnetic characterization technique. ,− …”

“…Here we consider two possible magnetization reversal mechanisms, coherent rotation and domain wall formation (Supporting Information, Section S6 and Figure S8). Recent studies have demonstrated a crossover of the magnetization reversal mechanism, in Co nanostructures, from (quasi-)­coherent rotation of macrospin to domain wall formation at a critical size. ,, While the energy barrier can be described as Δ E cr = KN for the coherent rotation, it is related to K as for the domain wall formation. Here σ is the area of the domain wall and A the exchange constant.…”

Increasing Magnetic Anisotropy in Bimetallic Nanoislands Grown on fcc(111) Metal Surfaces

“…Fitting the experimental data with the different models helps to elucidate the magnetization reversal mechanism. In this study, two magnetization reversal models have been considered for the simulations of the susceptibility, namely CR and DW; as we will see below, the island size and shape define which of the two takes place for a given island [ 35 , 36 , 37 ].…”

“…For the DW model, we have: , where , and and are the Co-Co and Co-4 d exchange stiffnesses. Additional parameters are the lattice parameter for the atoms in an island, taken identical to the Pt(111) one Å since the islands grow pseudomorphic with the substrate, the reduced exchange stiffness between Co atoms due to the lower dimensionality of the atoms in the islands pJ/m [ 35 , 38 ], and the pre-exponential factor in the Boltzmann term for the magnetization reversal frequency that we set to Hz.…”

“…For each island, we retain the smallest energy between the two reversal modes, and these dots are marked in green. The contribution of each island is then summed up, producing and curves, as described in our previous work [ 35 ]. The evaluation of the agreement between experiment and simulation was performed by visual inspection.…”